1. Field of the Invention
This invention relates to paper sizing compositions comprising a hydrophilic natural polymer such as a starch in combination with a polyester and improved water resistant papers sized therewith.
2. Description of the Prior Art
Surface sizing of paper using a tub or calendar means generally involves the application of dispersions of film-forming substances such as converted starches, gums, and modified polymers to an already formed paper or paperboard. Generally paper or paperboard is surface sized to control porosity, lay surface fuzz, improve printing ink pick resistance, and increase strength properties. Often paper or paperboard that is to be surface or tub sized contains internal sizing agents which regulate both the depth of penetration and the amount of the surface-sizing dispersion. While internal sizing is advantageous in providing resistance to water penetration throughout the paper or paperboard, surface sizing effects are generally confined to the external surface. The use of starch and glue as well as plant extracts as a surface size for paper apparently predates the discovery that improved water resistance can be imparted to paper with partially saponified rosin and alum as in internal sizing. With the advent of degraded, or modified, starches such as the alkaline-hypochlorite-oxidized starches and starch derivatives such as cationic starches, hydroxyethyl starch ethers and cyanoethyl starch ethers, renewed interest in the use of starch as a surface size occurred since these modified natural polymers effectively improve the surface properties of paper and paperboard and reduce the water sensitivity thereof. Because increased resistance to water is often necessary even with the use of such modified starches as size for paper and paperboard, particularly where such papers are used for off-set printing, it has been necessary to use mixtures of various polymers in dispersion form in admixture with the modified starches to impart increased water resistance to the surface sized paper and paperboard. One such size composition is disclosed in U.S. Pat. No. 3,931,422 wherein a surface-size composition including starch and a polyester of a polyhydric alcohol and trimellitic acid is utilized to provide improved "hold-out", or water resistance, to paper and paperboard. Because the so called "cationic" starches have a reduced biological oxygen demand, these modified starches have recently come into use as components of paper and paperboard sizing compositions as indicated by U.S. Pat. No. 4,029,885.